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In this study, a disk-type bluff body was installed at the upper part of a nozzle exit, and the circular jet inside the nozzle was controlled using a dielectric barrier discharge (DBD) plasma actuator (DBD-PA). The effects of the changes in the excitation frequency of the jet induced by the DBD-PA on the jet diffusion were elucidated. The experiments included visualization of the jet cross-section, particle image velocimetry analysis, and velocity measurements using an I-type hot-wire anemometer. When the DBD-PA was driven at a specific burst frequency (900–1400 Hz), a lock-in phenomenon occurred, in which the frequency of vortices generated in the initial jet coincided with the burst frequency. This lock-in phenomenon suppressed the merging of vortices by generating vortices at regular intervals. When vortex merging was suppressed, the jet was less likely to be entrained into the recirculation flow generated by the bluff body, thereby increasing the downstream jet width and average flow rate.

In this study, two dielectric barrier discharge plasma actuators (DBD-PAs) are driven at different times to control a circular jet flow. DBD-PA was installed inside a nozzle and on a disk-shaped bluff body above the nozzle. A vortex was generated in the jet using DBD-PA in the nozzle, and the development of the vortex was controlled using the DBD-PA installed in the disk-shaped bluff body above the nozzle. The two DBD-PAs were driven with a time lag based on the time the vortex reached the bluff body. The findings reveal that the vortices did not pair with each other when the DBD-PA of the bluff body was slightly delayed from the timing of the occurrence of the vortex on the circulating flow side. When the vortices did not pair, the flow was not entrained in the circulating flow, causing the jet width to widen. The vortices paired with each other when the DBD-PA of the bluff body was driven at the same time as or long after the occurrence of the vortex on the circulating flow side. When the vortex pair with each other, they collapse quickly after pairing, and the jet width narrows, as the flow is entrained in the circulating flow. The difference in jet width downstream of the bluff body was found by driving the two DBD-PAs installed inside the nozzle and on the bluff body at different times.

In this study, a double-tube burner nozzle with a DBD-PA (dielectric barrier discharge plasma actuator) and flame holder was employed to regulate the lifted flame. The flame holder held the lifted flame and the DBD-PA regulated the lifted flame to suppress the lifted flame’s vertical oscillation. To form a lifted flame, pseudo-biogas, a mixture of CH4 and CO2 in the ratio of 6:4, was jetted out through an inner nozzle and air through an annular nozzle. To control the air jet and prevent the lifted flame’s vertical oscillation, the DBD-PA was installed inside an annular nozzle. The DBD-PA was driven intermittently at regular intervals to produce a tangential flow in the same direction as the jetting gas, and the flame holder was placed above the nozzle outlet on the nozzle center axis. The finding indicated that by driving the DBD-PA at the slightly lower frequency than the frequency of the naturally shedding vortices in the free jet, the lifted flame’s vertical oscillation can be reduced by collapsing the jet’s vortex ring at a steady position and with a steady cycle. By impeding the lifted flame’s vertical oscillation, it is forecasted to reduce the blowout of the lifted flame and combustion noise.

The control of circular jets has been studied extensively in the fluid dynamics literature. Most studies introduce an entrance pipe upstream of the nozzle outlet to straighten the flow. In this study, we studied the free jet emitted from a circular nozzle that converges with the entrance pipe. We identified the cause of the naturally occurring vortex ring at the beginning of the jet, and we designed a mechanical intervention that suppresses the vortex. This experiment was performed at Reynolds number Re = 1400–5600. In the experiment, the visualized jet cross section and the flow velocity measured by the hot-wire anemometer were processed with a fast Fourier transform to determine the frequency at which the vortex ring arises. The frequency of the naturally occurring vortex ring depends on acoustic resonance inside the nozzle and is affected by the length of the entrance pipe. Also, the generation frequency of the vortex ring did not increase linearly with Re, but instead increased in discrete steps. A nozzle modification was also tested, with an acoustic material installed inside the nozzle to suppress pressure fluctuations caused by resonance inside the nozzle. This test was performed at Re = 2000, 5000, and 8000. The acoustic material suppressed the naturally occurring vortex ring, which reduced jet diffusion.

This study demonstrated jet control through the generation of an intermittently induced flow via burst-modulation from a coaxial type, axisymmetric nozzle shape DBD plasma actuator (DBD-PA). Here, a lock-in phenomenon occurred and was characterized by synchronization of the burst-frequency (frequency of burst-modulation) and the jet-generated vortex within a specific burst-frequency range, which aimed to clarify the change in the development process of the jet by the driving condition of the DBD-PA where the applied voltage and burst-frequency were varied. In the main text of this study the Reynolds number that is based on mean velocity and nozzle diameter of the jet was set at 5000. The applied voltages for DBD-PA were Vp-p = 16 kV, 14 kV, and 12 kV, while the burst-frequency was fburst = 0.2–2.0fn ( fn: frequency of occurrence of the initial vortex of free jet is 716 Hz). Results of the control demonstration showed that the lock-in phenomenon occurred at 0.9–1.4fn for all applied voltages. In the downstream, when the driving condition was Vp-p = 14 kV and fburst = 0.7fn, diffusion was dominant in all driving conditions due to fast collapse of the developed vortex ring. Moreover, such collapse was gradual with higher applied voltage. At the end of the burst-frequency of the lock-in phenomenon range, where the driving conditions was Vp-p = 16 kV and fburst = 1.4fn, diffusion became more suppressed as compared to the free jet, as the vortex ring near the nozzle collapsed due to vortices pairing, and did not develop. This work emphasizes that changing the applied voltage and the burst-frequency of DBD-PA potentially promotes and suppresses the diffusion of the jet.